Lohelin, Horn and Willerman found, in their study on adoptive children, that the level of the birth mother’s IQ had a significant effect on the average IQs of the adopted children, whereas the level of the adoptive families’ socioeconomic status (SES) did not, although there was about a 1-IQ point average difference in favour of the higher SES homes, which might let us think that a favourable environment gives genotypes more freedom to express themselves, whereas a very unfavourable one would probably be restrictive. They also found that the resemblance in IQ levels, even between biological children and their parents, clearly tended to diminish over the years as the child progressed into adulthood.
These findings are supported by Jensen, who states that ‘Apparently, as individuals progress from childhood to adulthood and encounter en ever-increasing range of experiences, they discover and select from their widening environment those aspects that are most compatible with their own genotypic proclivities. Therefore, with increasing maturity, the individual’s genotype is increasingly expressed in the individual’s phenotypic characteristics, reflected by a diminishing proportion of environmental variance and the increasing proportion of genetic variance.’ In Jensen’s opinion, indeed, ‘the perceptible environment is like a cafeteria’, i.e., people make different selections according to their genetic makeup, or genotype. Thus, the environment is not an inflexible structure but largely the person’s own creation.
Hence, genotypes and environments may be correlated: for instance, persons whose genotype is favourable for the development of a certain trait are more likely to grow up in an environment which is favourable to the development of the trait. This notion is know as genotype-environment (GE) covariance, and behavioural differences between persons that result from their self-fashioned environments are its phenotypic expression. GE covariance reflects the genetically driven differential selection of experiences from the available environment. It also includes the effects of differential treatment by parents, teachers, and peers, because their responses are largely evoked by the person’s distinctive genotypic character. As Jensen puts it, ‘The familiar phrase “nature and nurture” is now replaced by “nurture via nature”.’
So far, therefore, we have seen that most researchers concentrate their attention on the dual notion of nature and nurture. Some of them give nature supremacy over nurture, and some, like Jensen, argue for a conception which includes interaction between the two, where they mutually influence each other.
Very differently, a third group comes up which argues that the question should not be asked in terms of nature vs. environment: they criticise the presumption that heredity and environment are additive, separately acting causes whose contribution s to any characteristic can be neatly separated statistically. This, for Wahlsten and Gottlieb, is biologically unrealistic in view of all that is known today about the control of gene action and the interdependence of genetic and environmental effects: nature-nurture dualism as considered in behavioural genetics is invalid.
What is more, they argue that adoption method cannot separate the effects of heredity and environment because of the long-term importance of a shared prenatal environment. Prior to birth, the foetus lives in a milieu provided by its genetic mother, and adoption occurs too late in life to separate hereditary and environmental effects in the way they can be cleaved by ovarian grafting. Thus the adoption method is a good design for studying the importance of different postnatal environments, but cannot teach us about the role of genes.
The case of MZ twins reared apart is often considered the ultimate experimental method to grasp the extent to which a trait is genetically-determined. However, Wahlsten and Gottlieb argue, several kinds of correlated environments can occur for supposedly separated MZ twins. Because they are womb mates, their prenatal and early postnatal environments were shared. Also, because of the fact that adoption agencies practice selective placement, they are likely to share moderately similar environments in childhood.
Moreover, the unreliability of IQ measures is an important factor, as they note that it is well established that the IQ score of an adult can fluctuate by 20 points or more over a period of years.
Attempts to weigh the relative importance of genetic and environmental variations rely on their presumed additivity, and additivity demands separation and independence of causes, such that the contribution of genotype is not influenced in any way by the rearing environment and vice-versa. Such independence is far from evident, so, for Wahlsten and Gottlieb, genetic knowledge can help us to understand learning, memory, and intelligence only when a specific gene is identified and its relations with its surroundings are characterized, and not in terms of figures of generic heritability.
Going further in criticising the dual view of nature-versus-nurture debate, Bidell and Fischer argue in favour of adding a third element to the debate, namely, the active self-construction of the human being over himself. That constructive activity in intellectual development, they say, is neither illusory nor derivative (in the sense of being an outgrowth of more fundamental genetic and environmental processes). Instead, it is the central motivating force in the development of human intellectual abilities and the key to the interrelations of genome and environment. They warn against the ‘fallacy of reductionism, which artificially reduces multileveled, intrinsically related, self-organizing processes to dualistic abstractions like heredity and environment’. Such reductionistic separation leads to the elimination of the category that most fundamentally accounts for change in human cognitive development, i.e., the constructive activity of the person.
Thus behaviour should be treated as a product of self-regulating dynamic systems. Fischer’s ‘dynamic skills theory’ seeks to study developmental variability in person-in-context systems, allowing researchers to sort out the developmental relationships among genetic regulation, sociocultural activity and personal construction and thereby explain the epigenesis of intelligence.
Genetics and environment are seen as active self-organizing systems that interact with other self-organizing systems to determine intellectual development jointly. This perspective sees living systems as integrative systems, i.e. self-organizing systems in which the parts do not and cannot exist prior to or separately from one another or from the whole in which they participate.
The epigenetic conception of development implies that changes in cognitive, behavioural and emotional development into adulthood are not merely of a quantitative, but rather of a qualitative nature, i.e., there is emergence of qualitatively new features throughout the growing-up process. This conception is nowadays generally accepted, and the debate lies on finding out what mechanisms bring about those changes.
One position, known as predetermined epigenesis, considers that the emergence of new behaviours and mental capacities is performed according to the genetic-sequence programme. Thus, although external factors may switch on or off certain genetic instructions, the emergence of development changes is nevertheless driven by those instructions. The genetic code determines neurological structures independently of social or behavioural systems. To this view, Bidell and Fischer object that not only it reduces the role of the human agent to ‘a passive bystander in a predetermined genetic unfolding’, but also it is inconsistent with contemporary gene research because it depends on the linear model of genetic regulation, in which genes switch on at their appointed time and become the only causal agents in the production of the new structure or behaviour.
An alternative view considers the genetic system as one among many systems participating in and helping to regulate the production of that new structure or behaviour. A given developmental outcome is therefore free to vary depending on the systemic context, and not, as argued above, predetermined by the genes alone. This view, Bidell and Fischer call ‘constructive epigenesis’ ‘in order to emphasise the leading role of self-organizing activity in the emergence of qualitatively new structures and functions during development.’ Instead of genes and environment either singly or jointly determining cognitive outcomes, we, as human agents, determine our own cognitive outcomes as we actively make sense out of our world and build skills for participation in it. The goal of this approach is to use developmental variability to tease out the developmental relations among contributing systems at the biological, cognitive-behavioural and sociocultural levels of analysis. Thus, according to Bidell and Fischer, ‘research questions about how much of a given behaviour is due to genetics or environment […] are misleading because they direct attention away from the mechanisms by which cognitive skills are constructed’. I agree with this view so far as the calculation of heritabilities is concerned, even if we assumed that they are indeed fairly reliable, (which I think wouldn’t be very safe an assumption) to know that, for instance, IQ is about 70 to 78% inherited isn’t a very useful issue: first because IQ is a very debatable measure of intelligence, secondly because to know that it was inherited to that extent doesn’t have many practical applications (except in eugenic policies), instead, I think, it leaves a feeling of a certain helplessness of the individual condemned to his inheritance, whatever it be, whereas to know what mechanisms actually play in the construction of one’s intellect, beyond what is or not inherited, would have infinite practical applications, for instance in education or in dealing with persons with problems in cognitive development.
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Galton, F., History of Twins, 1875
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Intelligence, Heredity and Environment, ed. Stenberg, R. J. and Grigorenko, E.
Chapters: (1) Jensen, A. – The puzzle of nongenetic variance; (2) Loehlin, Horn and Willerman – Heredity, environment and IQ in the Texas Adoption Project; (3)Wahlsten and Gottlieb, The invalid separation of effects of nature and nurture: lessons from animal experimentation; (4)Bidell and Fischer, Human agency in the epigenesis of intelligence
